1. Field of the Invention
This invention relates to the measurement of wear on an end surface and particularly to the measurement of wear on the contact faces of a mid-span shroud in a gas turbine engine.
2. Description of the Prior Art
The turbofan engine is the type of power plant most widely used on large aircraft today. In the turbofan engine as distinguished as the turbojet engine, a portion of the working medium gases is pumped axially through one or more compression stages and is exhausted directly to the atmosphere without passing through the core portion of the engine. The compression stages which exhaust directly to the atmosphere are called fan stages and are generally positioned at the forward end of the engine. The ratio of air flowing through the fan stages to the air flowing through the core portion of the engine is termed the bypass ratio. Each fan stage is sized according to the bypass ratio requirements for the particular engine. In engines designed to have high bypass ratios the span of the fan blades is of such a substantial length that one or more mid-span shrouds between the root and tip of each blade is required for support of the blades. Even in engines of relatively low bypass ratio a mid-span shroud is commonly incorporated also at a position intermediate of the root and the tip of the blade.
The mid-span shrouds circumferentially support the blades to prevent flexural vibrations as well as excessive twisting of each blade near the blade tip. Each mid-span shroud is comprised of a plurality of segments which extend circumferentially from the pressure faces of the blades into abutting relationship with a corresponding number of segments which extend from the suction faces of the adjacent blades. Each suction segment and each corresponding pressure segment are urged into intimate contact by high centrifugal and gas pressure loads on the blades during operation of the engine. Aerodynamic distrubances and vibratory excitations produce continual abrasion on the contact faces of the abutting shroud segments.
A hard facing material is commonly applied to the contact face of each opposing shroud to inhibit surface wear. Notwithstanding the presence of hardfacing material, wear does occur and material must be periodically added to the contact faces to restore the shroud to its original dimension. Accurate measurement of the extent of wear after use and of the material buildup during repair is necessary to maintain the fan blades within the tolerances consonant with acceptable engine operation.